Sighting telescopes are used in hunting and for military purposes to aim weapons at targets great distances away. For that purpose these telescopes are fitted with a configuration of lens elements inside a housing to magnify a target view. In particular said configuration includes an objective and an ocular. The objective is a converging optical system to achieve a real target image and the ocular is a system of lens elements allowing the eye to look into said configuration of lenses.
An intermediate image projected from the objective into an objective-proximate image plane is reproduced in an ocular-proximate image plane. However the accompanying magnification substantially restricts the field of view and it is difficult to sight or observe close-by objects. To allow also sighting such objects, the state of the art employs variable magnification, i.e. zooming means. Moreover, the sighted object is laterally inverted in the an objective-proximate image plane as well as being upside down, and therefore requires correction.
To correct and magnify the inverted image, a reversing system is used within the sighting telescope. This reversing system allows an axially independent, respectively a defined displacement of two optical elements. Such optical elements include single lenses, cemented lens elements and reticles. In this manner an intermediate image projected into an image plane situated between the objective and the reversing system is erected and it is reproduced magnified in the image plane situated between the reversing system and the ocular where it is observed
Such lens element configurations however incur image defects. In the course of an object image reproduction into a virtual or real intermediate image, each lens element generates various aberrations, among which spherical aberration, defocusing, comas, field curvature, pincushion distortion, longitudinal and transverse color defects of different orders.
To correct such defects in the first image plane, the lens elements in the objective of a sighting telescope are combined and configured in a way that said defects shall be mutually compensating as much as possible over the path followed by the beam. Illustratively lens elements of flint glass and crown glass are cemented to each other to correct color defects.
Even when using optical means, a residue of image defects remains in the first image plane and these defects are very noticeable in high-zoom binoculars/sighting telescopes especially when magnification exceeds 4× in the high magnification ocular-proximate image plane. Transverse defects increase linearly with magnification and longitudinal defects increase in a square relationship.
The state of the art meets this problem by reducing these image defects by skillfully designing the lens element system to assure constant, good image quality across the full range of magnification.
High-zoom systems of the state of the art incur experience conflicts when correcting the magnified image defects at high magnifications and when correcting aid image defects at small magnifications over the entire field of view. When the system of lens elements is designed to compensate as well as possible the high-magnification image defects reproducing from the first image plane into the ocular-proximate image plane between the reversing system and the ocular, then clearly visible residual defects are incurred at low magnification, in particular coma and spherical aberration.
These image defects leave the user with the impression of low quality, especially as regards sparkle, steadiness and image sharpness.
Illustratively, a high-zoom sighting telescope is described in EP 1 746 451 B1. It describes a sighting telescope with a center tube configured between an objective and an ocular. The center tube receives a reversing system into which is integrated an adjustable magnifying optics. The magnifying optics consists of two mutually displaceable optical lens elements This reversing system is mounted between an objective-proximate image plane and an ocular-proximate image plane. By displacing the said lens elements, an intermediate image projected by the objective into the objective-proximate image plane is magnified in the an ocular-proximate image plane and in erected form. The maximum magnification is at least 4×.
Also, an optical beam deflecting unit is integrated into the reversing system. Said unit consists of an additional lens configuration situated on the reversing system's side facing the ocular and exhibiting a negative refraction between −20 dpt (diopters) and −40 dpt. This feature enhances the range of magnification. As a result, at all magnifications, a subjective sighting telescope field of view of at least 22° is assured at least for light of a wavelength of about 550 nm.
In addition the reversing system comprises a field lens spaced from the an objective-proximate image plane to allow a beam of rays from the objective and from an object point at the edge of the field of view being guided through the narrow duct of the reversing system.
The purpose of this field lens also is displacing the sighting telescope's range of magnification; this field lens is not primarily used for image defect correction.
Further and more developed sighting telescopes comprise a third, optical element in the reversing system (for instance U.S. Pat. No. 7,684,114 B2) in order to reduce image defects of high-zoom sighting telescopes, or they use aspheric lens elements in said reversing system. However a third displaceable optical element heightens the requirements of accurate guidance, entailing higher complexity as well as costs. Again the manufacture of aspheric lens elements is costly.
Accordingly the purpose of the present invention is to reduce the image defects across the full magnification range, in particular at the edges and also at lesser magnifications, the technical solution entailing little complexity and low costs. This sighting telescope shall be simple and easily handled and offer long service life.